This invention relates to apparatus for electrically heating pipes and particularly thawing frozen pipes, hydrants, electric cable ducts and the like.
In the past, welding machines have been used for thawing frozen pipes, often however with disastrous results. The main problem in using a welding machine for this purpose is that the output voltage varies with the size of load so that the voltage delivered can rise immediately to 100 volts or so if the circuit through the pipe is not a good one. This high voltage can drive the current over an alternative path which is usually a combination of ground wires and electrical distribution neutrals which are not designed to carry the resulting high current, resulting in fire hazards.
Machines designed specifically for pipe thawing have been developed and one such machine is described in Canadian Pat. No. 1,090,860 which issued on Dec. 2, 1980 to D. B. Hughes. The known apparatus comprises a movable trailer incorporating a generator, a step-down transformer and two bus bars to which electric cables can be connected. Each bus bar is arranged to accept several cables, one or more cables being connected to one end of a pipe to be thawed and one or more cables of the other bus bar being connected to the other end of the pipe to be thawed. To avoid excessive current in the cables, which gives rise to an inefficient voltage drop in the cables, excessive heating of the cables and other associated problems, the known apparatus includes means for sensing the current flowing in one or more of the cables and warning the operator to add on cables in parallel when the current sensed is excessive. Additionally, the generator is automatically tripped out, in case the warning is ignored.
While the above apparatus is an improvement over the welding machine and works extremely well under most circumstances, problems could still arise if the circuit through which the thawing current is flowing is not a good low resistance pipe circuit. In an extreme case the circuit would be open or, more commonly, the pipe circuit could have high resistance joints bypassed by a ground or neutral wire path of the parallel electrical distribution system. This latter case is not easily detectable because, if checked by an ohm meter, it would give the impression of a good conductive path. In reality, however, the neutral or ground wire is not of sufficient gauge to carry the large currents necessary for thawing. What can happen then is that, the operator, not knowing that he has a poor pipe path simply increases the voltage to attempt to obtain a sufficiently high current to thaw, and this can have the effect of damaging the neutral or ground wires in the service entrances of the neighbouring house. This can actually cause an immediate fire but more usually what would happen is that the insulation on the wire is damaged by heating, this damage not being readily apparent. However this can set the scene for fire or electrical shock occurring weeks or months later.
It is therefore a principal object of the present invention to overcome the above hazard.